Illness associated adenovirus infection is described in 15 immunocompromised patients. Patients were immunocompromised by severe underlying disease, immunosuppressive or corticosteroid therapy or by age (prematurity). Evidence of adenovirus infection was obtained by either viral isolation or, in two cases, characteristic adenovirus inclusion bodies at postmortem study. All clinical illness was associated with high fever (temperature greater than 39 degrees C). Eighty per cent of the patients had severe systemic complaints including malaise, lethargy, fatigue and night sweats; a similar number of gastrointestinal symptoms. Pulmonary complaints were described in 11 of 15 cases and included cough (67 per cent) and tachypnea (53 per cent). Roentgenologic evidence of pneumonia was demonstrated in 12 of 15 patients (80 per cent). Elevation of serum hepatic enzyme levels (serum glutamic pyruvic transaminase (SGPT)) occurred in eight of 11 patients (73 per cent) and was moderate to severe (serum glutamic pyruvic transaminase greater than 450 IU/liter) in five of 11 (45 per cent). Nine patients died; seven after a rapid downhill course and two after a prolonged illness. Evidence of adenovirus infection microscopically by autopsy in the lung, liver or both is demonstrated in four patients with fulminant systemic illness. Adenovirus infection should be considered in the etiology of severe overwhelming illness in the immunocompromised host.